As is well known to those skilled in the paper-making art, a web is dewatered by a pressing pressure applied in a press section of a paper-making machine. For example, it is known in the prior art to provide a web pressure-dewatering means and web transfer means, in the pressure section of a paper-making machine, in the form of a combination of a pair of press rolls with a pair of needled felts therebetween. More recently, a variety of means for imparting pressure onto a web have been provided with a combination of a single press roll and a shoe in order to improve the dewatering efficiency through an increase in the nip width, under application of a pressure, on the web. One example of the latter combination is known as a shoe-press assembly, whose simplified construction is illustrated in FIG. 8(a)
Referring to the prior art arrangement shown in FIG. 8(a), a shoe press assembly 20 includes a press roll 21 and a shoe 22, wherein the shoe 22 is shaped so as to conform to the press roll 21 along its circumference. In this way a nip width N under an applied nip pressure is large when compared to an arrangement employing a pair of press rolls. A shoe-press belt 23, that is constructed of a high-molecular elastic member, e.g., such as urethane, and a base cloth layer, runs between press roll 21 and shoe 22. A web P runs between shoe-press belt 23 and press roll 21, so as to be sandwiched between upper and lower needled felts 24 and 25. As a consequence, web P is press-dewatered, under the nip-pressure produced between press roll 21 and shoe 22, with the squeezed-out water migrating into upper and lower needled felts 24 and 25.
In this situation, however, as the water migrates into the upper and lower needled felts 24 and 25, they expand in sections where the felts are relieved of the nip pressure. As a result, the water accumulated in felts 24 and 25 actually migrates back into web P, by capillary action, thus rewetting the paper and causing a reduction in the efficiency of the dewatering process.
In order to solve the rewetting problem, a shoe press assembly 20', as illustrated in the prior art arrangement shown in FIG. 8(b), has been proposed, and which is disclosed in U.S. Pat. No. 4,483,745. In the shoe press assembly 20', a web P is sandwiched between a single needled felt 24 and a shoe press belt 23 and dewatered under application of a nip pressure produced by a press roll 21 and a shoe 22, such that the squeezed-out water migrates into needled felt 24. This arrangement still rewets the paper, since even though needled felt 24 is only a single piece, a portion of the water absorbed in sections of the felt having lower applied pressure still migrates back to the web.
Also in the prior art arrangement shown in FIG. 8(b), the web P of shoe-press assembly 20' and received on shoe press belt 23 has suffered from difficulties associated with paper transfer to and reception by a next step in the process. In particular, the surface of shoe press belt 23 is often highly polished so as to be very smooth. As a result, a water film of a uniform thickness is produced in a clearance between shoe press belt 23 and web P, whereby web P strongly adheres to shoe press belt 23 in the presence of the water, resulting in very poor paper release.
In order to solve the above-described problem, a shoe press belt 23, as shown in FIG. 8(c), is disclosed in Japanese Patent document No.: JP-A-94-57678. Here, shoe press belt 23 includes many recesses and protrusions formed on the surface of a web-receiving face 26b of a high-molecular weight elastic member 26, which is reinforced with a base member 26a. The recesses and protrusions are formed in such a way that high-molecular elastic weight member 26 is mixed, in advance, with particulate filler of a material of a higher hardness, such as kaolin clay, inorganic material, metal or the like, and in a following step, the web-receiving face 26b is polished off to expose particles of the particulate filler 27 on the surface. Shoe press belt 23 includes many recesses and protrusions, formed from particulate filler 27, on a surface of web-receiving face 26b. These recesses and protrusions are capable of breaking a wafer film formed between web P and web-receiving face 26b under application of the nip pressure, which decreases the adhesiveness between web P and shoe press belt 23 due to the presence of water. This arrangement enables transfer and reception of web P to and by the next step in the process.
This type of shoe press belt 23, which has web-receiving face 26b having many recesses and protrusions made by particulate filler 27 requires the technically hard steps of mixing particulate filler 27 into a non-processed material of a high-molecular weight elastic member; coat impregnating the non-processed material, consisting of high-molecular weight elastic member 26 mixed with the particulate filler 27, on and into a base member 26a; and curing the coat impregnated non-processed material. Further, the presence of particulate filler 27 creates an obstacle to controlling the global thickness of the belt after curing and polishing of the web-receiving face 26b to improve smoothness. In addition to this, while the polishing step is performed by pressing a polishing object onto the belt, which has been extended and driven between two rolls, many of the particles of particulate filler 27 that are exposed on web-receiving face 26b are hollowed out and fall off. Thus reducing the effectiveness of the particulate filler 27.